57 comments
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Well, if the tree was associated with a specific pollinating insect, one could argue the tree's defining characteristics reflect the preferences of that insect, and so it's evolution into that tree has been "selected for".

Of course, one then has to ask why that inect finds those features attractive and not others etc. etc.

Many North American trees are pollinated by wind. Flowers can be inconspicuous in these plants. We don't have Jacarandas in North Carolina, but I know that other plants in the Bignoniaceae family are pollinated by insects and birds. Color probably attracts the insects or birds. I don't know if they are particularly fragrant, but that would also be attractant. If they attract insects, they probably also provide nectar. I know hummingbirds and bees liked my Bignonia vine (that had orangey-red colored flowers).

And let's not forget that humans have probably been selecting the most ornamental plants for centuries (longer?). Even if that's not a particular cultivar (cultivated variety) it has likely been selected either vegetatively or by seeds from the best bloomers and the species itself has possibly changed.

I think the main difference from most other trees and bushes is that you're Canadian. Here is California there are plenty of native trees and bushes with brightly colored flowers. Now, if you restrict yourself to trees, you have a bit better case. Of all the sizeable trees, only buckeyes have showy flowers (at least, that I think of). The obvious adaptive explanation is animal pollination vs. wind pollination, but we are still left with the question of why some trees use one and some trees the other. Direct response to selection or simple inheritance? You could start by looking at the phylogenetic distribution of tree habit and animal pollination. That might provide some clues.

The Blue Jacaranda has been widely introduced into the US, Africa, Australia, etc. Its native land is Bolivia and Argentina. There it is visited and presumably pollinated by hummingbirds, bumblebees (Bombus) and other large bees (Eulaema and the likes). The 'explosive' flowering seems allow trees to be seen from afar. This may help reproduction, especially when trees are well spaced. Flowering synchrony is also important, for each individual tree may only flower for a week or two.Larry: There are many, many tropical Bignoniaceae and Leguminosae (among others) tree species with gaudy flowers. Canada is exceedingly depauperate and atypical with regard to evolutionary diversity. You should take a trip to the tropics with some 'adaptationists'.W. Benson

I've been in tropical rain forests and I don't recall seeing many trees covered with colorful flowers. Nevertheless, I bow to your expertise.

Since there are so many tropical trees that survive just fine without having to attract animal pollinators, what selective advantage does blue jacaranda have? And why didn't it take over the rain forest?

I think one has to be up in the canopy to see colorful flowers. I'd expect more white and flowers with scent attractants in the dim of the understory. I wouldn't expect too many wind pollinated plants in rainforests. I really can't think of too many tropical plants not pollinated by animals.

"And why didn't it take over the rain forest?", Larry, I am surprised to hear this from you. Perhaps I misread.

Thanks for the reminder, John. I haven't visited that site in a while and hadn't noticed there were plants. You are right that that's easier than searching papers--better with my schedule. Besides most papers have just a few bits of phylogeny, don't they?It would be nice if they'd update that site

And I just checked OneZoom TOL to see if they've gotten to plants yet and they seem to be working on it. I do hope they keep it up. Check it out here: http://www.onezoom.org/plants_Soltis2.htm It's still in Latin (but I like that--I don't know animals nearly as well as I do plants) and just to genus as far as I can tell. I can't wait until it is as complete as the other trees. I used the mammal tree with my high school bio class. Now they've put all the tetrapods together.

lynnwilhelm: Another good source for plant phylogeny is the plant phylogeny website (http://www.mobot.org/MOBOT/research/APweb/). It's technical as hell, but I enjoy browsing the trees and finding out relationships between things.

I see that the Bignoniaceae (including Jacaranda) is in the mint order (Lamiales), in a subgroup with unresolved relationships. Plants in that subgroup include Acanthus, Verbena, and the aquatic carnivorous bladderworts. A group can be "unresolved" because it hasn't been studied enough, or because it diversified really fast a long time ago, so it's hard to find genes that track the relationships well.

Larry, your comments about this tree are really surprising! Tropical forests have many mass-flowering showy trees. Lynn is absolutely right; virtually every tree in the rain forest is pollinated by active creatures seeking rewards. The pollinator diversity is staggering, and the floral diversity is also great, but far from random. Floral morphology of most trees is very specialized to attract a small subset of the total range of potential pollinators.

I strongly suggest that you spend more time in nature, preferably with an ecologist. You will quickly come to respect the adaptationist position. Your comments in this thread and others show that your emphasis on drift is not based on close examination of real nature.LJ

I'll take you at your word. I'm not familiar with tropical trees and it may be true that large numbers of them are covered with flowers at certain times of the year.

I know of several examples in temperate climates such as the lilacs in our back yard and cherry and apple trees.

Your other comment is bizarre. I'm well aware of the fact that field biologist have adaptationist explanations for every single shape and variety of flowers and leafs. That does not mean they can't be questioned. For example, what is the evolutionary advantage of specializing in attracting only a small subset of potential pollinators? Why not be a generalist or rely on the wind? Why don't all the palm trees in the neighborhood have colorful flowers? They seem to be surviving quite well without that selective advantage.

The evolutionary advantage of a specialized plant/pollinator relationship is that it works!Of course being specialized is only an advantageous adaptation if your pollinator is always around, if not...extinction. Adaptation fail.

I'm sure you would agree that a working adaptation does not mean it's the best solution.

I have just recently learned of this argument about selection vs. drift and I'm a bit stymied. It seems that both are important, but wouldn't selection always "win" when the environment changes? I do suppose that drift has been downplayed too much, but in my layman's eye I see that nature ultimately selects what works and what doesn't? And couldn't drift result in adaptations that could be selected?

Or is the argument about adaptationism? Because I try very hard not to look at evolution teleologically, I have a hard time understanding why some people would think that everything could be an "adaptation". As Barbara and others said in other comments, there could (should be) be a lot of chance involved in the adaptation process.

Barbara I am familiar with MOBOT as a good database for information on ornamental plants, I hadn't looked at what else they had. I have a horticultural background and need to start thinking more like a botanist to find out more. Thanks again.

I am fairly sure lynnwilhelm has hit it on the head. There are various "competing" selective pressures, and some of them may work over very long time horizons.

In the short run, it may be advantageous to evolve some costly signalling to get more females to mate with, in the long run it may doom your species.

In the short run, specialized pollination may be a good adaptation because it is more precise and you save costly pollen. In the long run, it is the generalists that survive because this or that pollinator group dies out, at least locally.

Definitely Jacaranda. A rainforest tree from South America. Rainforest tree species often have a reproductive strategy involving producing massive numbers of brilliantly coloured flowers all at once. The Australian flame tree, Brachychiton, is another example

PS They call it the blue jacaranda, if you need the common name. I'm afraid I have no adaptationist explanation for the colour of the flowers other than that already suggested by Rumraket (attractive to bees or humminbirds, or whatever pollinates jacarandas).

As a botanist, I have no idea what you consider so different about this species. Different in what way from what other species?

And if it is about the flowers, adaptationist explanations do make a lot of sense because a plant that does not get pollinated will not reproduce and die out.

If you want to talk about random walks in botany, go for leaf margins or leaf division (e.g., "provide an adaptationist explanation for the huge diversity of leaf shapes in the oak genus"). As long as the plants live in a well-watered habitats, the major selective pressure on leaves appears to be that they should be flat, but everything else looks like historical contingency plus stochastics.

Alex, there is often more to it than that, though as you implied, being well-watered does take away some of the pressure. Compound and serrated leaves have advantages in some drier climates. Leaf size varies regularly with rainfall and humidity. Leaf shape can be driven by insect predation (Passionflowers, for example). Leaf waxiness is typical of certain environments, across taxa. Fuzziness is also common across taxa in certain habitats. Deciduousness is common across taxa in seasonally dry or cold habitats. Even when contingency and history play a big role, part of that history is often due to past adaptations; for example, many compound-leaved trees that now live in wet forests belong to families that seem to have their evolutionary centers in drier forests, where compound leaves are favored.

I don't deny the role of drift and contingency, but I strongly disagree with Larry's insistence that so many morphological characters are due to drift. Looking closely at nature, that is not a reasonable inference.Lou Jost

But wouldn't leaf shape also be associated with light exposure? I know I have heard this, but I don't know if any work has been done on it. However, leaf arrangement and shape could maximize the amount of light reaching leaf surfaces. Then add in coatings, margins and overall shapes to maximize available water. It would of course be a compromise between maximizing light exposure and minimizing water loss where necessary.

I think you guys misunderstand. This is me, who LM would likely consider to be an "adaptationist", leaning over backwards to try to find something morphological in botany where it may makes more sense to assume drift than in flowers of all things.

Of course there are lots of selective pressures on leaves but all else being equal there really is no particular reason why a leaf should be serrate instead of crenate around the margins, for example.

Alex, it was your last sentence that seemed to bend too far towards drift. It is certainly not true as stated. But point taken, some leaf characters are better candidates for drift explanations than major floral characters. I don't think leaf margination is beyond selection, though. Different margins may have different abilities to dispose of guttation water, or shed dew or rain, or alter the duration of surface films of water on the leaf after rain, or deter specific herbivorous insects (who nearly always start eating at the margins), or cool more or less efficiently by affecting the laminar/turbulent flow regime of air passing over the leaf, or to imitate herbivory damage in order to attract visually-oriented parasites or predators of herbivores, etc. These are testable claims, either in the lab or by seeing whether certain habitats had an excess of one type of margination across families.LJ

Alex - You are incorrect about leaf margins. The nature of leaf margins is very strongly correlated with mean annual temperature (MAT). This his been a well-known fact ever since Bailey and Sinnott showed nearly a century ago that the percentage of leaves with toothed margins increases with MAT. It is a nearly perfect correlation and is probably because toothed margins have more area for photosynthesis early in the growing season. This is the basis for the widely used tool of Leaf Margin Analysis in paleoecology. Unfortunately, you chose one of the most well established cases of leaf morphology correlated with climate.

Sorry - I meant to write above that toothed margin leaves increase with DECREASING MAT. The point is that all of this is part of the botanical subfield of leaf physiognomy. You can find out more about LMA here for starters: http://www.smithsonianeducation.org/educators/lesson_plans/climate_change/

Ah, a fun set of thoughts followed by, “but then on the other hand . . . .” And thank you for explaining that the colorful flowers are the “anomaly”, for purposes of your question.

The question “Can you come up with an adaptationist explanation . . . ?” is kind of silly because of course I can. The more useful question would be, “Can you come up with an adaptationist explanation that seems to have some meaningful relationship with reality?”

This tree is obviously adapted to pollination by animals (probably large insects or birds). The size, shape, overall color, color pattern, scent, pollen presentation, nectar supply, etc., of the flowers are shaped by natural selection because these traits attract animals that transport pollen, necessary for fitness. Adaptation, adaptation, adaptation.

On the other hand, ancestral flowering plants were pollinated by insects. Colorful flowers are the default, so to speak. Perhaps what needs explanation here is why most of the trees you’re familiar with in Toronto have the small, green or brown flowers associated with wind pollination, not why this tree has big colorful flowers. (And then we may need to explain why/how some insect-pollinated plants like willows and the sedges Cymophyllus fraserisanus and Eriophyllum cringerum evolved from wind-pollinated flowering plants that evolved from insect-pollinated ancestral flowering plants, which evolved from wind-pollinated protogymnosperms – but let’s not go there just now.)

Once a tree has a relationship with an effective pollinator, selection will “improve” the tree, making it a more effective attractor of that pollinator. Adaptation.

However, what causes a particular pollinator to establish a relationship with the tree? There were probably many potential pollinators, and the few that ended up “selecting” the trees traits were those present in the ancestral tree’s environment, responsive to its traits, and most effective at transporting pollen. Many potential pollinators might have “worked.” The outcome isn’t due to chance alone, but it might have been different. And it may become different in the future, depending on the pollinators present in the habitats to which humans move these trees and the mutations that occur in the trees.

Assuming that the tree established a pollen-transporting relationship with animal species X, did it have to produce flowers of this exact color, shape and size? Probably not. The pollinator would probably have responded to other signals. In fact, it probably does respond to other signals and visits other plant species. The large purple flowers result from chance. Mostly chance. Chance constrained by what the ancestral trees could do and the behavior of their ancestral pollinators.

However, it is important that all the trees present the same signal, because pollinators will probably move on to another, similar tree. Odd ones are less likely to be pollinated. Adaptation, adaptation to a probably arbitrary standard.

Now this tree species has to respond to human cultivation, and may be very successful even if no effective pollinators visit it – a new wrinkle in its evolution.

And so forth, in a swirling sea of selection, adaptation, chance, and phylogenetic constraints.

Botany has come a long way in recent years. Contrary to what is implied in some of the comments here, we don't just make it up as we go along. Hypothesis of adaption or other constraints on floral morphology is a very active are of study, and is based on a rigorous phylogenetic framework, observations, and experiments. Nobody yet in this thread seems to have taken a look at the primary literature. Floral evolution is well studied in the family to which this plant belongs (Bignoniaceae). Start here:Pollination Biology of Jacaranda oxyphylla with an Emphasis on Staminode Function, Ann Bot. 2008 November; 102(5): 699–711. This is just one of many good studies of this kind.

I was just thinking that a look at the literature would be a good idea. I just searched "floral phylogeny" at my library and have a lot of reading I can do. (I've graduated recently and don't know how long I'll have access.)

"Nobody yet in this thread seems to have taken a look at the primary literature. " umkomasia, that's a bit presumptuous of you. I based my leaf morphology remarks on the literature. There is a large literature on how plant morphology relates to environmental constraints.LJ

But the answer to Larry's question may in fact have little to do specifically with the plant in question. It appears that the entire family has large, showy flowers, so the answer to how this species got to be that way could be just that it inherited a characteristic from its ancestors. What study of the species itself might tell us is how that characteristic is maintained by selection, not how it arose.

I can't name the tree but it makes a creationist case.Rather then seeing mutation and selection creating new species it would be better to see trees etc as just taking advantage of opportunity to thrive in RICH areas.So originally , after the great flood, the world was so rich in climate, like the amazon today, that easily the slightness difference in a typr of tree etc would breed and thrive within feet of its parents. Not desperate mutationism but easy minor differences in pffspring getting rich options.The biblical creationist can see massive adaptation as a result of a original great climate and diversity created by great options to bredd within minor changes from birth. I think diversity however is also from innate triggers greatly changing bodies.

And did trees get planted in the hold in twosies because they were "Unclean?" Or did they get planted in sevensies? Or did the seeds float on the flotsam and jetsam of the flood until the waters receded and germinate where they laid?

lutesuiteIts not evolution by selection on mutation plus time.Its just wealth and health of migrating flora. Every niche is taken by minor differences in offspring. no mutations needed.Yes there must be and is other mechanisms for great changes.I am confident within years or decades land creatures became the marine creatures we know and love and eat today.I am confident of many great morphological changes even quicker. Finally I'm confident that all the present, or almost, differences in mankind were already here within a century or two after the flood.So no slow evolution but serious triggering of innate biological mechanisms yet undiscovered.We all need mechanisms to explain the diversity.

It seems that trying to find the tree that grew in the front yard of my childhood home is next to impossible when I'm is looking for it; it appears only when not!

The pictured example is a little different from the species (variety?) that I knew: This has multiple trunks converging at the ground. Mine had a single, solid trunk and vast canopy. But, of course, lots of the same sticky, purple flowers that ended up as a purple carpet on the ground.

Here's my "adaptationist" explanation for why this tree is so different from most other trees and bushes:

They are different for the same reason house cats and domesticated dogs are different from most other felines and Canidae: Because people really like them for all the reasons peculiar to humans! Maybe it's the perfect combination of evergreen, filtered light and abundant, perfumed flowers? We keep planting them (or not cutting them down), so they keep growing and reproducing -- naturally, in varieties we tend also to prefer.

Alexander,The form of the tree could be affected by how it is grown. Multitrunk or single stems can be created by a nursery for the trade. Multiple trunks can also result for damage to a tree which caused multiple shoots to grow from the base.

Can you come up with an adaptationist explanation for why this tree is so different from most other trees and bushes?

That's a curious formulation, which I would have read without surprise on ENV. It's like saying "why doesn't everything have a long neck, if it's so great being a giraffe?".

The basis of any adaptive explanation in a sexual species is relative to alleles in an ancestral population, not in unrelated (or at least, more distantly related) species. As others have noted, the specific conditions of its ancestors, and the role of human selectors, will have a significant influence. And constraint and contingency, of course.

My own tendency would be to assume adaptation as the 'default' explanation in the matter of flowers - nectar, scent, shape, colour and pattern in the visible and the uv are all typically adaptive in insect-pollinated species. One could, of course, point to these same features in asexual species. But since these invariably have sexual ancestors, relatively recent, this does not disbar an adaptive explanation for their origin.

Can you come up with an adaptationist explanation for why this tree is so different from most other trees and bushes?

I don't know whether to call this parochialism, naivite, or ignorance.Question: why should anybody pay attention to bloviations about biological evolution from somebody who doesn't know the first thing about organisms?

Laurence A. Moran

Larry Moran is a Professor Emeritus in the Department of Biochemistry at the University of Toronto. You can contact him by looking up his email address on the University of Toronto website.

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Quotations

The old argument of design in nature, as given by Paley, which formerly seemed to me to be so conclusive, fails, now that the law of natural selection has been discovered. We can no longer argue that, for instance, the beautiful hinge of a bivalve shell must have been made by an intelligent being, like the hinge of a door by man. There seems to be no more design in the variability of organic beings and in the action of natural selection, than in the course which the wind blows.Charles Darwin (c1880)Although I am fully convinced of the truth of the views given in this volume, I by no means expect to convince experienced naturalists whose minds are stocked with a multitude of facts all viewed, during a long course of years, from a point of view directly opposite to mine. It is so easy to hide our ignorance under such expressions as "plan of creation," "unity of design," etc., and to think that we give an explanation when we only restate a fact. Any one whose disposition leads him to attach more weight to unexplained difficulties than to the explanation of a certain number of facts will certainly reject the theory.

Charles Darwin (1859)Science reveals where religion conceals. Where religion purports to explain, it actually resorts to tautology. To assert that "God did it" is no more than an admission of ignorance dressed deceitfully as an explanation...

Quotations

The world is not inhabited exclusively by fools, and when a subject arouses intense interest, as this one has, something other than semantics is usually at stake.
Stephen Jay Gould (1982)
I have championed contingency, and will continue to do so, because its large realm and legitimate claims have been so poorly attended by evolutionary scientists who cannot discern the beat of this different drummer while their brains and ears remain tuned to only the sounds of general theory.
Stephen Jay Gould (2002) p.1339
The essence of Darwinism lies in its claim that natural selection creates the fit. Variation is ubiquitous and random in direction. It supplies raw material only. Natural selection directs the course of evolutionary change.
Stephen Jay Gould (1977)
Rudyard Kipling asked how the leopard got its spots, the rhino its wrinkled skin. He called his answers "just-so stories." When evolutionists try to explain form and behavior, they also tell just-so stories—and the agent is natural selection. Virtuosity in invention replaces testability as the criterion for acceptance.
Stephen Jay Gould (1980)
Since 'change of gene frequencies in populations' is the 'official' definition of evolution, randomness has transgressed Darwin's border and asserted itself as an agent of evolutionary change.
Stephen Jay Gould (1983) p.335
The first commandment for all versions of NOMA might be summarized by stating: "Thou shalt not mix the magisteria by claiming that God directly ordains important events in the history of nature by special interference knowable only through revelation and not accessible to science." In common parlance, we refer to such special interference as "miracle"—operationally defined as a unique and temporary suspension of natural law to reorder the facts of nature by divine fiat.
Stephen Jay Gould (1999) p.84

Quotations

My own view is that conclusions about the evolution of human behavior should be based on research at least as rigorous as that used in studying nonhuman animals. And if you read the animal behavior journals, you'll see that this requirement sets the bar pretty high, so that many assertions about evolutionary psychology sink without a trace.

Jerry Coyne
Why Evolution Is TrueI once made the remark that two things disappeared in 1990: one was communism, the other was biochemistry and that only one of them should be allowed to come back.

Sydney Brenner
TIBS Dec. 2000
It is naïve to think that if a species' environment changes the species must adapt or else become extinct.... Just as a changed environment need not set in motion selection for new adaptations, new adaptations may evolve in an unchanging environment if new mutations arise that are superior to any pre-existing variations

Douglas Futuyma
One of the most frightening things in the Western world, and in this country in particular, is the number of people who believe in things that are scientifically false. If someone tells me that the earth is less than 10,000 years old, in my opinion he should see a psychiatrist.

Francis Crick
There will be no difficulty in computers being adapted to biology. There will be luddites. But they will be buried.

Sydney Brenner
An atheist before Darwin could have said, following Hume: 'I have no explanation for complex biological design. All I know is that God isn't a good explanation, so we must wait and hope that somebody comes up with a better one.' I can't help feeling that such a position, though logically sound, would have left one feeling pretty unsatisfied, and that although atheism might have been logically tenable before Darwin, Darwin made it possible to be an intellectually fulfilled atheist

Richard Dawkins
Another curious aspect of the theory of evolution is that everybody thinks he understand it. I mean philosophers, social scientists, and so on. While in fact very few people understand it, actually as it stands, even as it stood when Darwin expressed it, and even less as we now may be able to understand it in biology.

Jacques Monod
The false view of evolution as a process of global optimizing has been applied literally by engineers who, taken in by a mistaken metaphor, have attempted to find globally optimal solutions to design problems by writing programs that model evolution by natural selection.